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Viruses

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Page 1: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Viruses

Page 2: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Characteristics of Viruses

• Viruses are NOT alive• They do not have a cytoplasm or organelles • Cannot carry out cellular functions • They do not divide (reproduce) outside host

cell

Page 3: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Viral Size and Structure

• Viruses are super small! • The shape of a virus is the

result of its genome and protein coat – capsid

• Some viruses have a bilipid membrane called an envelope that surrounds the capsid

Page 4: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide
Page 5: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Classification of Viruses

• Viruses can be classified by:– if the have RNA or DNA – If genome is single stranded or double stranded– The nature of their capsid– Presence or absence of envelope

Example: SARS is a single-stranded, linear RNA and envelope virus with lollipop-shaped proteins that make the envelope look like a crown

Page 6: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Viral Replication• Viruses need a host cell! • They can spread from host to host through the

air, water, food, or bodily fluids (yuck) • Viruses infect both prokaryotes AND

eukaryotes

Page 7: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Viral Replication• Viruses need to recognize a proper host cell

before they can infect it. • Viruses are obligate intercellular parasites –

they replicate only by using host cell enzymes and organelles to make more viruses

Page 8: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Viral Replication Pathway

“lock-and-key” fit between proteins and receptor molecules of host cells

A viral infection begins when a virus enters the host cell.

The viral genome takes over the metabolic machinery of the cell and makes new

viruses

Page 9: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Replication in DNA Viruses

• When DNA viruses enter a host cell, it makes mRNA, which is the template for making proteins

• Other DNA viruses insert their DNA into the host cell’s chromosome– This inserted viral DNA is known as a provirus – The host cell’s enzymes then transcribe the

provirus into mRNA and create viral proteins

Page 10: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Replication in RNA Viruses

• The genome of some RNA viruses enter host cells and acts as mRNA

• They are then translated into new viral proteins! Yikes!

• Retroviruses contain the enzyme reverse transcriptase in addition to RNA

Page 11: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Retroviruses• Reverse transcriptase uses RNA as a template to

make DNA which then inserts into the host cell’s genome.

• Reverse transcriptase reverses the normal process of making proteins

https://www.youtube.com/watch?v=73QarmyHHtw

Page 12: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Replication in Viruses That Infect Prokaryotes

• Bacteriophages (phages) are viruses that infect bacteria

Page 13: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

The Lytic Cycle• During the lytic cycle, a virus invades a host

cell, produces new viruses, and ruptures (lyses) the host cell – Lysis is the process of phage particles bursting

from the bacteria cell and destroying it!

• Viruses that reproduce only by the lytic cycle are virulent. – Virulent viruses destroy the cell that they infect

Page 14: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

The Lytic Cycle

Page 15: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

The Lysogenic Cycle• Allows viruses to hid in their host cell for days,

months, or years! • These viruses are called temperate viruses

Phage DNA that is integrated into a specific site of the host cell’s chromosome is called a prophage

Page 16: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

The Lysogenic Cycle

Page 17: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Viruses: Tools for Biotechnology

• They are important research tools! We can replace large pieces of DNA of a phage with DNA of a particular interest!

Page 18: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

The Origin of Viruses• Most scientists believe viruses evolved from

early cells– One hypothesis is that the first viruses were

probably naked pieces of nucleic acid that could travel from cell to cell

– The viruses entered cells through damaged cell membranes

– Over time genes evolved that coded for protective protein coats as well as signaling for specific cells

Page 19: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

The Origin of Viruses• Evolution by Natural Selection!• Think of some examples!

Page 20: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Vectors of Viral Disease

• Because viruses are lifeless particles, their spread depends on other agents.

• A vector is an intermediate host that transfers a pathogen or a parasite to another organism

Examples of vectors for viruses: animals, mosquitoes, ticks, fleas, YOU!

Page 21: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Vectors of Viral Disease

• The West Nile virus, (causes fever, headache, coma, paralysis, and even death) infects mainly birds.

• If a mosquito bites a bird

infected with West Nile virus, then bites a human, the virus can spread!

Page 22: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Examples of Viruses in Humans

• flu• Chickenpox• Measules• Polio • Viral hepatitis • Cold• Yellow fever • West Nile • And much much more

Page 23: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Viruses and Cancer• Cancer is the result of: • Some viruses contain viral oncogenes, genes

that cause caner by blocking the normal controls on cell reproduction

Uncontrolled Cell Growth

Page 24: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Viruses and Cancer• Other viruses cause cancer because the viral DNA

inserts itself into a host’s chromosome near a proto-oncogene, which usually controls cell growth.

• The proto-oncogene is converted to an oncogene.

https://www.youtube.com/watch?v=Cn502md6laE https://www.youtube.com/watch?v=aC-jnY2t1Wc

Page 25: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Emerging Viral Diseases

• Emerging diseases are illnesses caused by new or reappearing infectious agents that typically exist in animal populations –often in isolated habitats- and can infect humans who interact with these animals

Example: Recent Ebola outbreak blamed on a bat infecting a human

Page 26: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Chickenpox and Shingles pg. 489

Tamar

Page 27: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

VIRAL HEPATITIS PG. 489

Benny

Page 28: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Acquired Immune Deficiency Syndrome

(AIDS)Pg. 490Hadar

Page 29: Viruses. Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide

Short Essay• Vaccinations • Vector Control and Drug Therapy • Emerging Viral Diseases